AAC encoder breaking 5.1 LFE channel

Description

Summary of the bug:
For a long time now, I've been doing some encodings into x265/aac using ffmpeg.
Recently I started noticing a problem with the audio channels that get - I think - mixed up.

The original file media info would show:Front: L C R, Side: L R, LFE

But the encoded file will show:Front: L C R, Side: C, Back: L R

I'm no expert, but I think the LFE channel is either dropped or converted.
I think this behaviour started around December 2017.

To be certain that something indeed changed, I encoded a file twice, used two static builds (from here: ​https://johnvansickle.com/ffmpeg/), once using a build from 2017-09-02, and one from 2018-01-24.

I attached to the tickets the logs of both encodings, the media info (+ the original).
You can download the original mkv file, as well as encoded files from there: ​http://176.9.126.245/encoding-results/

You can clearly see in the mediainfo of 2017 that the channels were correct (with LFE), but that's not the case in the 2018 encoding.